In PHP, string written in single quotes is displayed 'as it is'. Variables and most of escape sequences are not interpolated.
$name = 'Farhan'; echo 'Welcome $name'; // Output: 'Welcome $name';
Whereas in double quotes, variables and escape sequences are interpolated.
$name = 'Farhan'; echo "Welcome $name"; // Output: "Welcome Farhan"
Thanks @cseder for correcting to use
interpolated instead of